The present invention relates to a method for manufacturing hardened machined parts, and more particularly to a method for manufacturing nitrided machined parts from steel particularly for use under light load conditions.
It has been the usual practice that important steel parts which are manufactured by working steel material are subjected to a surface hardening treatment. In this case, when the surface hardening treatment is carried out by a nitriding process this process is superior to a carburizing process in that it gives higher hardness and does not cause severe strain on the parts during processing as is usual in a carburizing process. Therefore, it is very desirable that machine parts such as gears, cylinders, etc. which require absolute accuracy in dimension are subjected to a nitriding treatment since this can do away with the process necessary for correction of size after the nitriding treatment takes place. However, in conventional steel, when its core portion is given the required hardness prior to the nitriding treatment, its surface becomes so hard that machining is made impossible. On the other hand, when it is hardened to just a degree that allows easy machining, the hardness required for the machine part can not be obtained. In addition, conventional steel has a very poor nitriding ability so that it cannot make use of the advantageous characteristics of the nitriding treatment as above-mentioned.
In view of this, the inventors of the present application have previously proposed an invention entitled "Nitriding Steel of the Precipitation Hardening Type" as Japanese patent application Ser. No. 135293/1973. This steel is originally aimed at steel which has a core hardness above 350 Hv which had not been attainable in conventional nitriding steel until then. Accordingly, the hardness of this steel at the stage of being subjected to a solution heat treatment cannot be decreased below 280 Hv; in reality, therefore, such a high level of hardness making the effective machining of parts for use under low load conditions, e.g. parts for automobiles which are to be mass produced, impossible. That is, for example, when a usual gear which is commonly mass produced is taken into consideration, the gear cutting process is difficult unless the hardness of the gear blank is below 250 Hv. Consequently, as steel for such mass produced machine parts, one which has a hardness below 250 Hv at the stage of subjection to the solution heat treatment has to be aimed at. Of course, since this level of hardness is necessary for machining, the level of hardness after subsequent subjection to the nitriding treatment should be higher to give the strength to the machined part required of a machine part. A level of hardness of 280 to 300 Hv is satisfactory for parts produced in large quantity as above described, and a corresponding surface hardness above 650 Hv is sufficient.